The user stack is dependent on the user address environment. As the
process exits, this address environment is destroyed anyway, so the
stack does not need to be released separately.
There is also an issue with this when the process exits via exit(). The
problem is that the task group is released prior to this "up_release_stack()"
call along with the address environment, and trying to free the memory
either causes an immediate crash (no valid addrenv), or frees memory into
another process' heap (addrenv from a different process).
Signed-off-by: Ville Juven <ville.juven@unikie.com>